1. Technical Field
The present invention relates to a light source device, an image display apparatus, and a monitor apparatus.
2. Related Art
In projection type image display apparatuses in recent years, in general, a discharge lamp such as a super-high pressure mercury lamp is used as a light source. However, such a discharge lamp has problems in that durable life is short, instantaneous lighting is difficult, a color reproducibility range is narrow, and an ultraviolet ray radiated from the lamp deteriorates a liquid crystal light valve. Therefore, a projection type image display apparatus including a laser light source that irradiates monochromatic light is proposed instead of such a discharge lamp. The laser light source does not have the problems described above but is disadvantageous in that the laser light source has coherence. Consequently, an interference fringe appears as speckle noise and an image is deteriorated on a projection surface on which a laser beam is projected. Therefore, in order to display a high-definition image, measures against the speckle noise are necessary.
As means for removing speckle noise, plural light-emitting elements having center wavelengths slightly different from one another in design are provided in array. This makes it possible to reduce speckle noise by obtaining a wider spectrum band compared with that obtained when one light-emitting element is used (see JP-T-2004-503923 term “JP-T” as used herein means a published Japanese translation of a PCT patent application).
The means for removing speckle noise described in JP-T-2004-503923 is devised on the premise that a light source that does not need an external resonator structure, i.e., a light source that directly outputs laser beam is used. Indeed, in the case of the light source that does not need the external resonator structure, there is an effect that speckle noise is suppressed.
In the case of a light source including an external resonator, basic components are a light-emitting element and a wavelength selection element (a resonator mirror). Even when plural light-emitting elements are used, taking into account cost and easiness of assembly, it is a general practice to use a wavelength selection element that selects a single wavelength. In the wavelength selection element, for laser oscillation, it is necessary to narrow a band of a wavelength to be selected. As a result, as described in JP-T-2004-503923, even if wavelengths of lights emitted from the array light sources are varied, a single wavelength is selected by the wavelength selection element. Therefore, coherence of the light source as whole including the light selection element does not decrease.